In the art of producing silver halide photographic emulsions, it is generally desired to achieve further improvements in the sensitivity of photographic emulsions. Chemical sensitization with a sulfur compound or combination of a gold compound and a sulfur compound has been prevailingly employed as a useful method for enhancing the sensitivity of a silver halide photographic emulsion.
In addition, it is universally known that when a silver halide photographic emulsion is chemically sensitized by both a gold compound and a sulfur compound, the inherent sensitivity of the photographic emulsion can be further increased by a combined use with a nitrogen-containing heterocyclic compound, such as a hydroxytetrazaindene compound or the like. Details thereof are described, e.g., in British Patent No. 1,315,755, etc.
However, when such a nitrogen-containing heterocyclic compound such as hydroxytetrazaindene or the like is added to a photographic emulsion upon chemical sensitization using both a gold compound and a sulfur compound, though the processing dependence (e.g., development temperature dependence, etc.) is greatly improved along with the enhancement of sensitivity, it creates problems in that pressure desensitization occurs to a significant extent in photographic films to which pressure has been applied before exposure, and deterioration of an aptitude for safelight (i.e , generation of fog by exposure to safelight) occurs.
The quality of images subject to impairment by, e.g., pressure desensitization, can be improved using known means. For instance, means are known so as to make it impossible for applied pressure to effectively reach the silver halide grains, e.g., a method of incorporating a plastisizer such as polymers, emulsions, etc., a method involving lowering the silver halide to gelatin ratio in the silver halide emulsion, and so on.
More specifically, there have been disclosed a method of using heterocyclic compounds in British Patent No. 738,618, a method of using alkyl phthalates in British Patent No. 738,637, a method of using alkyl esters in British Patent No. 738,639, a method of using polyhydric alcohols in U.S. Pat. No. 2,960,404, a method of using carboxyalkyl celluloses in U.S. Pat. No. 3,121,060, a method of using paraffin and carboxylates in Japanese Patent Application (OPI) No. 5017/74 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application), a method of using alkylacrylates and organic acids in Japanese Patent Publication No. 28086/78, and so on.
However, the method of using plasticizer is restricted as to the amount of plasticizer used because the plasticizer lowers the mechanical strength of the emulsion layers. On the other hand, increasing the content of gelatin reduces the speed of development processing, and so on. Thus, both of these methods are difficult to use so as to fully achieve their desired effects in view of the foregoing defects, but furthermore they are also unable to achieve any improvement in the above-described deteriorated aptitude for safelight.